1. Field of The Invention
The present invention relates to a liquid metal cooling type fast reactor and more particularly to a fast reactor core having an improved structure or arrangement of core constituting elements.
2. Related Prior Art
A fast reactor core is generally composed of a number of fuel assemblies each loaded with fissionable or fissile material and utilizes liquid sodium (Na) as a coolant for removing heat from the fuel.
In a usual and steady operation of the fast reactor, although temperatures of respective portions of the fast reactor core do not abnormally rise, the core is designed on an assumption of an accident so that the core can be safely shut down even in a case where the temperature rises over predetermined values.
For example, in a case where the sodium in the fuel assembly is heated to a high temperature and the density thereof is lowered and, in a the case of a small-sized core, since a large number of neutrons leak from the core, a negative reactivity is inserted to thereby safely shutdown the core. However, in a case of the large-sized core, a reduced amount of the neutrons leaks from the core and the reactivity is made positive when the sodium in the fuel assembly is highly heated. In such case, it is required to analyze, in detail, in consideration of the other factors of the reactivity, whether or not the core can be shutdown. Accordingly, it is extremely worthwhile for a safety design of the core to make negative the reactivity effect, that is, the temperature coefficient of the coolant due to the temperature increase of the sodium.
However, in general, it is not desired in a power generation plant to extremely reduce the output power of a core from a viewpoint of power generation cost. In the meantime, in a conventional design of the core, in order to make the core reactivity negative in the case of sodium temperature increase, it has been required to make the core power less than about 100 MWe. However, this causes a problem that the temperature coefficient of the coolant becomes positive when the core is designed for the large power generation. That is, the design of the core for the large power generation renders the size of the core large. In a case where the density of the coolant is reduced by increasing the temperature thereof, the temperature coefficient (reactivity) of the coolant is decided by the interrelationship between the positive reactivity effect induced by the decreasing of neutron slowing down, the increase in the neutron generation number in the plutonium fuel, and the negative reactivity effect induced by the enhanced leakage of the neutrons from the core. For a large reactor core, the latter-mentioned effect is very small near the central portion of the core because of less leakage of the neutrons. Due to this the reactivity is made positive in the vicinity of the central portion of the core, so the reactivity is made positive in the entire core even if the temperature of the core is uniformly increased.
As a main reason for the abnormal increase of the temperature of the coolant in the core, there are considered a power increase (power increasing type phenomenon) due to a positive reactivity insertion and a coolant flow rate reduction (flow loss type phenomenon) due to a pump operation stop. Accordingly, at the time of the flow loss type phenomenon, the pressure of the coolant is simultaneously reduced in accordance with the reduction of the flow rate, but in a prior art technology, there has not been provided a positive and appropriate countermeasure to such phenomenon or accident.